Decolorization of kraft wood pulp bleach effluents

ABSTRACT

Pulp mill bleach effluents are decolorized, after a suitable lime treatment, by acidifying to a pH of about 1-5 and thereafter passing the acidified effluent through a bed or mass of a macroreticular adsorbent resin. The macroreticular adsorbent resin may be aromatic or aliphatic in character and has a porosity of at least 10 percent and a specific surface area of at least 10 square meters per gram.

llnited States Patent Paleos [4 1 M111. 2%, 1972 [54] DECOLORIZATION 0F KRAFT WOOD 3,509,121 4/1970 Benko ..210/36 x PULP BLEACH EFFLUENTS 3,531,370 9/1970 Gould ..162/33 3,531,463 9/1970 Gustafson ..210/24 X [72] Pale, Lansdale' 3,412,018 11/1968 Monzie ..210/21 [73] Assignee: Rohm and Haas Company, Philadelphia,

Pa. Primary Examiner-Michael Rogers Assistant Examiner-Thomas G. Wyse [22] Flled' 1969 Attorney-G. W. F, Simmons, C. A. Castellan and A. M. [21] Appl. No.: 871,975 Esterlitz 521 US. Cl ..210/27, 162/30, 210/30, [57] ABSTRACT 210/40 Pulp mill bleach effluents are decolorized, after a suitable lime [51] Int. Cl ..B01d 15/04 treatment by acidifying to a PH of about 1 5 and thereafter [58] Fleld 0i sEfllCh ..162/16, 30, 32, 33; 210/24, p g the acidified effluent h g a bed or mass of a 210/28 40 macroreticular adsorbent resin. The macroreticular adsorbent 56 R t C1 d resin may be aromatic or aliphatic in character and has a 1 e erences l e porosity of at least 10 percent and a specific surface area of at UNITED STATES PATENTS least 10 square meters per gram.

3,120,464 2/1964 Berger et al. ..162/33 12 Claims, No Drawings DECOLORIZATIION OF KRAFT WOOD PULP BLEACH EFFLUENTS This invention relates to the decolorization of wood pulp bleach effluents, particularly, Kraft wood pulp bleach effluents. More particularly, the invention relates to the decolorization of such effluents by utilizing a macroreticular aromatic or aliphatic adsorbent resin.

As is well known, in bleaching pulp produced by the Kraft art, a process is employed which involves treatment with free chlorine, neutralizing with caustic soda and then bleaching with calcium hypochlorite or chlorine dioxide. The chemical treatments separate the lignins and tanins in the wood from the cellulose fiber and they are ultimately carried out with the wash water. Large quantities of water are used in the bleaching and pulping of wood fibers resulting in a product of large volumes of highly colored waste streams. The pulping industry has long been plagued with such waste streams, and particularly, with the problem of colored liquid effluents from the bleaching process, especially where such effluents are discharged into streams or waters serving as municipal or industrial water sources. This problem is becoming more acute as more and more governmental bodies adopt and enforce stricter pollution laws. (The Kraft process is well-known and is described, for example, in Shreves Selected Process Industries published by McGraw-l-lill Book Company, Inc. 1950, beginning at page 638.)

The present invention provides an effective method for removing, to a degree not heretofore possible, the color bodies from the bleaching effluent or spent bleaching liquors. In accordance with the known techniques the color bodies may ultimately be burnt in a recovery furnace along with the Kraft black liquors. The color bodies are referred to hereinafter as organic color bodies inasmuch as they appear to be carboxylic in nature or origin. However, the use of the term organic color bodies" is not to be taken to exclude the possibility of inclusion of minor amounts of other color bodies, e.g., inorganic color bodies or constituents.

it is further known that pulp bleaching effluents result from a chlorination step and an alkaline extraction step. For ease of reference, and consistent with general trade usage, these bleach effluents will be referred to as chlorination effluents which are typically yellow in color and as alkaline or caustic extraction effluents or filtrates which are typically dark brown in color. The art to date has primarily used a lime treatment of the bleach effluents, particularly the caustic extraction effluent, to reduce the color of the effluent. While this technique has merit it has certain disadvantages including the fact that very large quantities of lime must be used, and further, the degree of color removal is not nearly as effective as that of the present invention. For example, the caustic extraction filtrate, which remains after filtering or separating the lime from the caustic extraction effluent, is brown or dark brown in color.

ln a preferred embodiment of the invention, the caustic extraction effluent stream which normally has a pH of about 9 to l l is first given a 2 percent lime treatment to reduce the color of said stream from 2,000 to 500 color units on the Co-Pt color scale. The lime reduces the color bodies by flocculation and/or precipitation of said bodies. This is then followed by a filtration step to dewater the effluent slurry. The resulting still brown colored filtrate is then mixed with the chlorination effluent which has a pH of about 2-3 and the mixture adjusted to a pH of about 2-4. The color of this mixture is an intense yellow and has an optical density of about 0.43 to 0.6 measured at a wavelength of 500 mu (millimicron) or an optical density of about 0.2 to 0.6 measured at 560 mp. Optical Density is measured on the Spectrophotometer Beckman DU instrument. An optical density of 0.1 measured at 500 mp. corresponds to about 60 color units on the cobalt-platinum (Co- Pt) scale and an optical density of 0.] measured at 560 my. corresponds to 86 color units on the Co-Pt (or Pt-Co) color scale. This intensely yellow colored effluent mixture is then passed through a bed of an aromatic or aliphatic macroreticular adsorbent, of the type more fully described below, wherein virtually all of the remaining color can be removed. The overall schematic process (except for the regenerating and/or burning steps described hereinafter) is shown in the diagram below:

Caustic extraction efliuent pH 9 to 10 Flow (1 to 1.5 volume units/day) Filtration In an example, the caustic extraction filtrate and chlorination filtrate are mixed at approximately a 1-5 ratio after filtering the excess lime. Loading of this stream onto a mass or bed of macroreticular resin adsorbent in a column is performed at a flow rate between 1.0 and 2.0 gallons per cubic foot per minute. The macroreticular adsorbent in this example is a synthetic insoluble cross linked polystyrene polymer or resin used in the form of 20-50 mesh beads and used in a column operation well known in the art. The adsorbent polymer has a surface area of about 330 square meters per gram, a porosity of about 0.44 ml./ml. and an average pore diameter of about to Angstrom units. The optical density (0.550) of the infiuent is measured at an adjusted pH of 7.0 using a Beckman DU at 560 mu. Bed volume fractions are collected periodically and the optical density of these fractions is measured to obtain percentage color removal and percentage leakage data. A typical exhaustion cycle showing effluent optical density and percentage color removal and leakage values is given in Table I below. Average color removal as calculated on the basis of the influent color is about 80 percent after the bed has been in extended use. During its initial use the bed removes as much as 96 percent or more of the color. The balance, or color not removed, is reported as percent leakage.

TABLE [Typical Loading Cycle of a Lime Treated Caustic Extraction Filtrate combined with Chlorination Filtrate Optical Density readings were obtained using a Beckman DU at 560 my. on samples adjusted to pH 7.

RV bed volume(s) Regeneration of the macroreticular adsorbent used in the example above can be accomplished in a number of ways. Ordinarily, most effective regeneration is accomplished with dilute solutions of sodium hydroxide (about 0.2 percent NaOH to 1 percent NaOH or higher). An additional advantage of the present invention is the ability to utilize as the regenerant various in-plant streams. For this purpose one can use as regenerants streams known in the art as White Liquoror Weak Wash, analyses of which are given below in Table II. A further advantage of the present invention is the convenient elimination of the color disposal problem, i.e., the regenerant stream, after regeneration of the exhausted macroreticular adsorbent resin, which will then contain the color bodies," can be burnt, either alone, or together with excess white liquor and/or with black liquor in a suitable furnace. Also, a very substantial portion of the caustic can be recovered from the furnace, if desired, and reused as a regenerant. Thus, the regenerant may be essentially a nocost item.

TABLE II Analysis of Effluent Streams used for Regeneration of Macroreticular Resin Adsorbent White Liquor Weak Wash g/L g/L Total Na,O 92.6 20.3 NaOH as Na O 62.5 13.0 Na,S as Na,O 18.7 5.1 Nu,CO, as Na,O ll.3 2.8 NaOH as NaOH 80.6 16.7 Na,S as Na,S 23.6 6.4 Na,CO, as Na.,CO I94 4.8

The macroreticular resins employed as the adsorbents herein are not new compositions of matter in themselves. Any of the known materials of this type are suitable. For example, there may be used granular cross-linked polymers containing from 2 to 100 percent by weight of units of one or more polyethylenically unsaturated monomers such as macroreticular resins prepared by suspension polymerization of polymerizable ethylenically unsaturated molecules comprising from 2 to lOO weight percent of at least one poly(vinyl) benzene monomer preferably divinylbenzene, trivinylbenzene, alkyldivinylbenzenes having from one to four (C to C alkyl groups substituted in the benzene nucleus, or alkyltrivinylbenzenes having one to three (C to C )-alkyl groups substituted in the benzene nucleus, or a mixture thereof. Besides the homopolymers and copolymers of these poly(vinyl)benzene monomers, one or more of them may be copolymerized with up to 98 percent (by weight of the total monomer mixture) of (l) monoethylenically unsaturated monomers, or (2) polyethylenically unsaturated monomers other than the poly(vinyl)benzenes just defined, or (3) a mixture of l and (2) and still result in a suitable macroreticular resin. In order to produce the high porosity and high specific surface areas required of the resins in the present invention, suspension polymerization procedures well known in the art may be employed, for example, the procedures disclosed and claimed in British Pat. No. 932,126.

Examples of the suitable alkyl-substituted diand tri-vinylbenzene monomers are the various vinyltoluenes, the divinylxylenes, divinylethylbenzenes, 1,4 divinyl 2,3,5,6 tetramethylbenzene l,3,5 trivinyl 2,4,6 trimethylbenzene, l,4 divinyl,2,3,6 triethylbenzene, 1,2,4 trivinyl 3,5 diethylbenzene, and 1,3,5 trivinyl 2 methylbenzene.

Examples of other suitable polyethylenically unsaturated monomers referred to above are: divinylpyridine, divinylnaphthalenes, diallyl phthalate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, divinylsulfone, polyvinyl or polyallyl ethers of glycol, of glycerol, of pentaerylthritol, of monothio or dithio-derivatives of glycols and of resorcinol,' divinylketone, divinylsulfide, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl succinate, diallyl carbonate, diallyl malonate, diallyl oxalate, diallyl adipate, diallyl sebacate, divinyl sebacate, diallyl tartrate, diallyl silicate, triallyl tricarballylate, triallyl aconitate, triallyl citrate, triallyl phosphate, N,N'- methylenedimethacrylamide, N,N'-ethylenediacrylamide, trivinylanphthalenes, and polyvinylanthracenes. Alternatively, the macroreticular cross-linked polymer may comprise essentially all aliphatic materials, for example, it may comprise 2-100 percent by weight of units of trimethylolpropane trimethacrylate, the balance preferably comprising a polar monomer such as an acrylate of the type mentioned below, acrylonitrile, etc.

Examples of the suitable monoethylenically unsaturated monomers referred to above are: methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, tertbutyl acrylate, ethylhexyl acrylate, cyclohexyl acrylate, isobornyl acrylate, benzyl acrylate, phenyl acrylate, alkylphenyl acrylate, ethoxymethyl acrylate, ethoxyethylacrylate, ethoxypropyl acrylate, propoxymethyl acrylate, propoxyethyl acrylate, propoxypropyl acrylate, ethoxyphenyl acrylate, ethoxybenzyl acrylate, ethoxycyclohexyl acrylate, and the corresponding esters of methacrylic acid, ethylene, propylene, isobutylene, diisobutylene, styrene, vinyltoluene, vinyl chloride, vinyl acetate, vinylidene chloride and acrylonitrile. Polyethylenically unsaturated monomers which contain only one polymerizable ethylenically unsaturated group, such as isoprene, butadiene, and chloroprene, are also to be regarded as falling within the category of monoethylenically unsaturated monomers.

The preferred proportion of the polyethylenically unsaturated cross-linking monomer is from 8 to 25 percent by weight of the total monomer mixture from which the resin is prepared. Suspension polymerization usually produces the resin in the form of granules or beads having an overall bead size in the range of about 0.1 to about 3 millimeters average diameter. The bead form of the resin is quite useful for the adsorption process of the invention. In this process the material or substance being separated or concentrated is adsorbed on the surface of the resin particles and the effectiveness of the process depends on the presence of a high ratio of surface area to weight of resin.

The macroreticular resin used in the process of the invention is preferably of 16 to mesh (US. Standard Screen Series) in particle size, but for some special purposes the resin particles may be as small as about 400 mesh (37 microns).

Macroreticular resins are characterized by the presence throughout the polymeric matrix, ofa network of extra-gellular micro channels or pores. While these micro channels are very small, they are large in comparison with the poresin in conventional homogeneous cross-linked gels. Macroreticular resins suitable for use in the invention may have specific surface areas of up to 2,000 sq. m. per gram or more.

In general, since the macroreticular resins find their greatest value in the processing of aqueous solutions, they are preferably not extremely hydrophobic or water-repellent. The preferred resins are cross-linked resins which have solubility parameters (expressed in the units Calories cu it: can meter of at least 8.5 and those having such parameters up to 15 or more are satisfactory for use in aqueous systems. The substantially non-ionogenic macroreticular cross-linked synthetic resin further has a porosity of at least 10 percent, a specific surface area of at least 10 sq. in. per gram and which is not appreciably swollen by the medium, so that the substance is adsorbed on to the surface of the resin, the substance being then, if desired, desorbed from the resin. Preferably the macroreticular adsorbent resin will have a surface area in the range of about 50 to 1,000 sq. meters per gram with a more preferred range being about 100 to 500 sq. meters per gram. The average pore diameter is also of some significance and it should range of from about 50 A. to about 1,000 A., and more preferably from about 75 A. to about 400 A. The physical properties of the macroreticular adsorbent resins, for example, porosity, surface area, pore size, etc., may be determined in accordance with standard techniques practiced in the art; for example, see pp. 153-167 of the book entitled Oxidation- Reduction Polymers by Cassidy and Kun, copyright 1965, published by Interscience Publications, New York, New York.

As noted hereinabove, for most effective results, the pulp mill bleach effluents are acidified prior to the decolorization treatment by the macroreticular resin adsorbent. This can be most conveniently and economically accomplished by adjusting the ratio of the caustic extraction stream and the chlorination effluent stream so that the mixed bleach effluent prior to passage through the macroreticular resin adsorbent is adjusted to a pH range of about 2 to about 4. However, the pH range can be expanded to about 1 to 5. Of course, the effluent streams need not be necessarily mixed to effect the acidification adjustment. That is to say, one can add acid, for example, hydrochloric acid or sulfuric acid, to adjust the pH of the mixed effluent to a range of about l5, and more preferably, 2 to about 4, prior to passage through the macroreticular adsorbent.

ln addition to the Kraft or sulfate pulp bleach effluents, the process of the present invention is also applicable to the sulfite bleach effluents which have similar properties and which also contain objectionable amounts of organic color bodies.

I claim:

l. in the method of decolorizing pulp mill bleach effluents by removing organic color bodies therefrom, the bleach effluents comprising at least one caustic extraction effluent which has been treated with lime to remove at least a portion of the color bodies, the improvement which comprises acidifying the caustic colored filtrate after separation from the lime, to a pH of about 1-5 and thereafter passing the colored filtrate through a mass or bed of a substantially non-ionogenic marcoreticular adsorbent resin thereby removing substantially all of the remaining organic color bodies, said resin being in the form of small heads, the resin having a porosity of at least percent and a specific surface area of at least 10 sq. meters per gram, and said resin not being appreciably swollen by the medium being treated.

2. Method according to claim 1 in which the caustic extraction filtrate is acidified to a pH of about 2-4 by mixing with a colored chlorination effluent and thereafter passing the combined filtrate and effluent through the macroreticular adsorbent resin.

3. Method according to claim 1 wherein the macroreticular adsorbent resin is regenerated with an alkali or alkaline metal hydroxide solution to remove the color bodies from the adsoraromatic macroreticular resin.

9. Method according to claim 7 wherein the adsorbent is an aliphatic macroreticular resin.

10. Method according to claim 6 wherein the regenerant stream after regenerating the exhausted adsorbent resin by removal of the color therefrom is then burned in a furnace.

11. Method according to claim 8 wherein the adsorbent resin is an insoluble crosslinked polystyrene in the form of about 20 to 50 mesh beads, said resin having a surface area of about 330 sq. meters per gram, and a porosity of about 0.44 mL/ml. (44percent) and an average pore diameter of about 85 to 95 angstrom units.

12. In the method of decolorizing pulp mill bleach effluents by removing organic color bodies therefrom, the improvement which comprises acidifying the pulp mill bleach effluents to a pH in the range of about l5 and thereafter passing the pH adjusted colored effluent stream through a mass or bed of a substantially non-ionogenic macroreticular adsorbent resin to effect decolorization of the effluent stream, said resin being in the form of small beads, the resin having a porosity of at least 10 percent and a specific surface area of at least 10 square meters per gram, and said resin not being appreciably swollen by the medium being treated.

lOlO29 055$ 

2. Method according to claim 1 in which the caustic extraction filtrate is acidified to a pH of about 2-4 by mixing with a colored chlorination effluent and thereafter passing the combined filtrate and effluent through the macroreticular adsorbent resin.
 3. Method according to claim 1 wherein the macroreticular adsorbent resin is regenerated with an alkali or alkaline metal hydroxide solution to remove the color bodies from the adsorbent.
 4. Method according to claim 3 wherein the regenerant is sodium hydroxide.
 5. Method according to claim 3 wherein the regenerant is a byproduct stream from the pulp mill plant which contains an alkali or alkaline metal hydroxide.
 6. Method according to claim 5 wherein the regenerant is a ''''white liquor'''' or ''''weak wash'''' stream.
 7. Method according to claim 1 wherein the macroreticular adsorbent resin has a surface area in the range of about 50 to 1, 000 sq. meters per gram.
 8. Method according to claim 7 wherein the adsorbent is an aromatic macroreticular resin.
 9. Method according to claim 7 wherein the adsorbent is an aliphatic macroreticular resin.
 10. Method according to claim 6 wherein the regenerant stream after regenerating the exhausted adsorbent resin by removal of the color therefrom is then burned in a furnace.
 11. Method according to claim 8 wherein the adsorbent resin is an insoluble crosslinked polystyrene in the form of about 20 to 50 mesh beads, said resin having a surface area of about 330 sq. meters per gram, and a porosity of about 0.44 ml./ml. (44percent) and an average pore diameter of about 85 to 95 angstrom units.
 12. In the method of decolorizing pulp mill bleach effluents by removing organic color bodies therefrom, the improvement which comprises acidifying the pulp mill bleach effluents to a pH in the range of about 1-5 and thereafter passing the pH adjusted colored effluent stream through a mass or bed of a substantially non-ionogenic macroreticular adsorbent resin to effect decolorization of the effluent stream, said resin being in the form of small beads, the resin having a porosity of at least 10 percent and a specific surface area of at least 10 square meters per gram, and said resin not being appreciably swollen by the medium being treated. 